It is not known how any mitogenic peptide or tumor promoter activates a quiescent cell. In 1978, we reported evidence that certain peptide growth factors act by rapidly increasing Na permeability of the quiescent cell. Transformed cells in culture apparently contain more Na and/or are more permeable to Na than non-transformed ones. The greater permeability to Na probably causes the increase in Na-K pump activity which occurs soon after mitogenic activation and, at least in part, the greater pump activity in some transformed cell lines. Recently we reported that the enhancement of Na-dependent amino acid transport, which is also highly correlated with the activation of the quiescent cell in vitro, may in turn be due to the hyperpolarizing effect of pump stimulation since the pump is electrogenic. We plan to further explore the hypothesis that an increase in cell permeability to Na plays an important role in transformation, tumor promotion, and mitogenic signal transduction. Secondary changes in free Ca or protons may be responsible for altering energy metabolism and macromolecular syntheses. We call this hypothesis stimulus-division coupling since the proposed alterations in cation transport are analogous to the ones involved in muscle contraction and secretion. The activity of the Na specific and non-specific cation transporters will be assayed as a function of cell density and growth state in non-transformed and transformed fibroblasts, by following both Li and 22Na ion entry and exit. Effects of internal and external Ca and MG ions, purified plasma membranes, individual peptide mitogens and tumor promoters, e.g., vasopressin and phorbol esters, will be examined. Amiloride-derivatives will be tried as irreversible inhibitors and photo-affinity labels of the amiloride-sensitive Na transporter. Growth of fibroblastic and two mammary carcinoma cell lines, in completely defined media, will be used to look for specific antagonists of the mitogenic response to vasopressin, phorbol esters and other tumor promoters. Na transport inhibitors, retinoids, selenium, Li and Rb ions will be tried. The mechanism and regulation of Na and Ca transport will be studied in the MCF-7 and ZR-75-1 lines of mammary carcinoma cells and in purified membrane vesicles from fibroblastic cells.